3 Answers
3

If portrait images are in the clear majority then these need to be displayed as clearly as possible.

Rotating the image to show it on it's side is not a good experience so showing it in the correct orientation is to be preferred. If the user can click on the preview and see the full sized image without going into the full details then there is no real loss of functionality.

We make the thumbnails very small (70px in the largest dimension), and then allocate a 70px square for the thumbnail in the UI (in which the thumbnail is centred). We've found in practice that many users identify the documents they're searching for as much (if not more) by its orientation and shape as by its content. Your option (a) breaks the user's sense of the document's orientation (I'm looking for a landscape document), and your option (b) implies an incorrect scale (the landscape document looks to be a significantly smaller paper size than the portrait document; something like an A5 document versus an A4 one in ISO 216 paper sizes).

I also encourage you to experiment with much smaller thumbnails. Even with office documents that are mostly just text, once you're below about 128px-square (on our regular machines) there's not really an appreciable difference between 96px-square thumbnails and 70px-square thumbnails.

+1 Kit, this was actually another idea I had. The design I'm provided with requires a fairly sizeable thumbnail, one that's almost legible. I will keep this in mind for future designs, though! Thanks!
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msanfordMar 15 '12 at 2:54

I don't believe this is a good solution for this problem. We also produce software that allows users to select a document from a list of thumbnails. By showing the thumbnail in its actual orientation and in full, users are much quicker at finding the document they're looking for. With small thumbnails, the shape and orientation of a document provides at least as much of a hint (perhaps even more) than the content of the thumbnail itself.
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Kit GroseMar 14 '12 at 23:43